In recent years, projectors have been in common use for making presentations in meetings, conventions and so on. Also, in households, the projectors are commonly used for home theatres.
Such projectors display enlarged projections of an original image by modulating light projected from a light source, in accordance with image information provided. As a light source for such projectors, lamps with high luminance and high color rendition and which are close to a point light source are used. One example of such light source is a high-pressure mercury lamp.
In detail, such lamps have an arc tube inside in which a lighting-emitting substance, mercury of no less than 200 [mg/cm3] for example, is enclosed and a pair of tungsten electrodes are disposed substantially opposite to each other. Further, the arc tube encloses, in addition to mercury, a halogen material. The halogen material prevents tungsten composing the electrodes from diffusing and adhering to the inner wall of the arc tube and causing the wall to blacken by causing the known halogen cycle reaction during lighting,
In high-pressure mercury lamps, it is important to restrain the occurrence of the so-called “arc jump” phenomenon. When the arc-jump phenomenon takes place, arc spots, the part of the electrodes from which the arc discharge in between the two electrodes originates, move in an undesired and instable manner. Since the arc jump phenomenon results in flickering on a projected image, its occurrence should be restrained to the utmost.
As a means of restraining the arc jump phenomenon described above, Patent Document 1 discloses a technique of increasing an absolute instantaneous value of an alternating current supplied to the lamp immediately before polarity reverse takes place. According to this disclosure, by locally heating the arc spot on the electrode immediately before the polarity of the electrode reverses from positive to negative, electrons will be more likely emitted from the arc spot after polarity of the electrode has reversed to negative. By repeating this process for each half-cycle period of the alternating current, the arc spot can be stabilized.
Further, Patent Document 2 suggests a technique of detecting a distance between the two electrodes and reshaping a lamp current in accordance with the distance detected. (More precisely, what is detected is a lamp voltage, which indicates the distance between the two electrodes.) In high-pressure mercury lamps, it is commonly known that protrusions at the tips of the electrodes grow and abrade (or in other words, the lamp voltage decreases and increases) depending upon the total lighting time. Patent Document 2 discloses that a continuous increase of the lamp voltage can be suppressed by applying the technique disclosed therein. The abnormal growth and abrasion of the protrusions at the tip of electrodes leave the discharge arc in an unstable state. Patent Document 2 seeks to suppress such growth and abrasion by a switching of an alternating current waveform.